1. Technical Field
This invention relates to the art of making catalytic composites for treating automotive emissions, and more particularly to three-way catalyst compositions that have high conversion efficiency for hydrocarbons, carbon monoxide, and nitrogen oxides.
2. Discussion of the Prior Art
Precious metals, including palladium and rhodium, have been used as active catalyst materials in automotive emission devices. However, palladium by itself as the dominant or sole catalyst material is limited in its effectiveness to promote the reduction of nitrogen oxide compounds at high temperatures and is not optimum in promoting oxidation of hydrocarbons at such temperatures. An example of such palladium usage is disclosed in U.S. Pat. No. 4,123,391. A variety of cost-increasing alternatives have been used to augment the capabilities of palladium, primarily by the use of other precious metals, particularly rhodium. Rhodium has been deployed with platinum, or with platinum and palladium, because rhodium is selective in the reduction of nitrogen oxides by carbon monoxide into innocuous nitrogen in the presence of excess air in the air to fuel ratio (A/F) supplied to the catalyst. The presence of excess oxygen in the exhaust will be hereinafter referred to as "lean exhaust" and encompasses a lambda of 1.1-8.0. Platinum is readily sintered at elevated temperatures in a lean exhaust atmosphere, thus reducing available surface area of a catalyst. Lean exhaust conditions also have a detrimental effect upon the rhodium because it interacts with gamma alumina and diffuses thereinto and thus results in a reduction in activity, probably due to a loss of rhodium accessibility to the exhaust system (see U.S. Pat. No. 4,675,308).
This invention has discovered that the addition in a unique manner of small amounts of titania and rhodium to a palladium/lanthana catalyst will synergistically enhance the total catalytic effect for oxidation and reduction, particularly under lean exhaust conditions. Titania has not been used heretofore as a catalyst enhancing ingredient nor has titania and lanthanum oxide been used together as catalyst enhancing ingredients. Titania has been incorporated heretofore in catalyst support materials for purposes of stabilizing such support materials, but prevented from entering into the catalytic process by coverings of other catalytically active ingredients (see U.S. Pat. Nos. 4,504,598 and 4,123,391). In some cases, titania has been layered on the support in large amounts prior to being covered with a catalytically active material (see U.S. Pat. No. 4,350,613). Straight titania has been slurried onto prevalent precious metal materials, including palladium, in a continuous film (of about 0.003-0.01 inches) to make an oxidation catalyst more lead-tolerant (see U.S. Pat. No. 4,650,782). Such catalyst construction was not intended for enhancement of reduction processes, the titania did not function as a reducing catalyst, and there was found no special catalytic synergism between titania and palladium.
Lanthanum oxide has been suggested for use as a substrate stabilizer as well as other rare earth oxides (see U.S. Pat. Nos. 4,624,941 and 4,283,308). In such disclosures, lanthanum oxide is dispersed as a sintered powder onto a substrate support such as alumina; the lanthanum oxide and support together are then covered with one or more noble metals. No improvement in catalytic activity was attributed to the presence of lanthanum oxide. U.S. Pat. No. 4,791,091 found that dispersing lanthanum oxide onto alumina in small particle size allowed less interference with the activity of the precious metal when compared to dispersing lanthanum oxide in larger particle sizes, but failed to appreciate how lanthanum oxide could effect a synergistic improvement between a precious metal and a nonprecious metal catalyst.